Mirror of the SDK's deepagents.backends.protocol.FILE_NOT_FOUND sentinel.
Hardcoded here rather than imported because file_ops is on the TUI import
path and importing deepagents at module scope violates the
startup-performance rule (see AGENTS.md); this module is dependency-free.
test_file_ops.py::test_file_not_found_matches_sdk is the drift guard that
fails if the SDK renames the sentinel.
